Method of preparing silver halide emulsions and image processes usingsame

ABSTRACT

Method of purifying an aqueous emulsion containing a silver halide suspended therein and using same in an imaging process which comprises contacting said emulsion with a first ionexchange material which provides one of the ions selected from the class consisting of hydrogen ions and hydroxyl ions and with a second ion-exchange material which provides the opposite ion from that provided by said first ion-exchange material, the time of contact of said emulsion with each of said ion-exchange materials being sufficient to exchange substantially all of the ions in said emulsion for hydrogen and hydroxyl ions, and then separating the emulsion from said ion-exchange materials. The emulsion is applied to a base support exposed to actinic radiation to selectively harden a portion of the surface and then washing away the unhardened layer.

llnited States Patent [1 1 Arena 1 June 12, 1973 REGENERANT TANK A CID FOR CATION EXCHANGE RES/N METHOD OF PREPARING SILVER HALIDE EMULSIONS AND IMAGE PROCESSES USING SAME [75] Inventor: Ronald D. Arena, Durham, Conn.

[73] Assignee: Addressograph-Multigraph Corporation, Mount Prospect, Ill.

[22] Filed: Apr. 22, 1971 [21] Appl. No.: 136,571

[51] Int. Cl G03c 5/00 [58] Field of Search 96/94, 114.7;

[56] References Cited UNITED STATES PATENTS 992,898 5/1911 Payne 96/36 2,240,116 4/1941 Holmes.... 210/37 2,484,647 8/1945 Roberts 210/37 2,663,640 12/1953 Reichel et al... 96/36 3,458,439 7/1969 Schmidt 210/37 INFLUENT- TOP COVER PROVIDED WATER WITH AN AUTOMATIC PRESSURE RELIEF COLD MANUAL VENT VALVE H07 i 5 PRESSURE GAUGE] AIR INLET N t (/00 P515) I PRESSURE 1mm i CARTRIDQE FILTER Primary Examiner-Norman G. Torchin Assistant Examiner-R. L. Schilling Attorney-Sol L. Goldstein [57] ABSTRACT Method of purifying an aqueous emulsion containing a silver halide suspended therein and using same in an imaging process which comprises contacting said emulsion with a first ion-exchange material which provides one of the ions selected from the class consisting of hydrogen ions and hydroxyl ions and with a second ionexchange material which provides the opposite ion from that provided by said first ion-exchange material, the time of contact of said emulsion with each of said ion-exchange materials being sufficient to exchange substantially all of the ions in said emulsion for hydrogen and hydroxyl ions, and then separating the emulsion from said ion-exchange materials. The emulsion is applied to a base support exposed to actinic radiation to selectively harden a portion of the surface and then washing away the unhardened layer.

6 Claims, 1 Drawing Figure REGENERANT TANK- CAUSTIC FOR ANIONEXCHANGE RESIN REACTION IGTTLE 1 COLLECTOR METHOD OF PREPARING SILVER HALIDE EMULSIONS AND IMAGE PROCESSES USING SAME BACKGROUND OF THE INVENTION This invention relates to silver halide emulsion coatings and to the preparation thereof. More particularly, this invention relates to an improved procedure for desalting silver halide emulsions to remove soluble cations and anions.

In the production of silver halide gelatin emulsions, insoluble silver halides are precipitated in hydrophilic colloidal suspensions, such as gelatin emulsions, by a metathetical reaction between a soluble silver salt and a soluble halide salt. During the production of the insoluble silver halide by metathesis, there is formed a soluble salt which provides undesirable cations and anions in the silver halide emulsion. These cations and anions must be removed before the silver halide emulsion is used in photography.

Silver emulsion of the light-hardenable type which are known in this area as wash-off emulsions in addition to the silver halide containing cross linking agents which cause the emulsion, e. g. gelatin, to become water insoluble when exposed to actinic radiation. Typical cross linking agent is K CrO It is known that the time of exposure of these silver emulsions in order to attain proper cross linking is dependent in part on the purity of the gelatin employed. Therefore, gelatin of a particular purity had to be employed in order to assure producing light-hardenable emulsions that would have a reasonably fast response to actinic radiation. Surprising results have been obtained by passing the sensitizing emulsion through the ion exchange columns which permitted the use of a wider range of colloids that heretofore were deemed unusable.

I-Ieretofore, soluble salts have been removed from silver halide emulsions by conventional washing techniques, but these have been objectionable because they are slow, tedious and inefficient. The principal washing methods which have been used involve gelling, coagulation of flocculation, or dialysis. In the latter technique permeable membranes are used. Numerous other desalting methods are reported in the literature and patents. For example, Glafkidas (Photographic Chemistry, Foundation Press, London, 1958, Vol. I) mentions centrifuging, sedimentation, coagulation, and dialysis. Most conventional methods are slow and inefficient. In all of these procedures, hours and sometimes days are required for processing, and huge volumes of water are involved which often create a good deal of material handling problems.

GENERAL DESCRIPTION OF THE INVENTION In accordance with this invention, colloidal emulsions containing silver halides are purified by treatment with a combination of cation-exchange and anionexchange material whereby waste byproduct salts and other ionic materials are quickly removed. This procedure is highly suitable for desalting various silver halide emulsion systems at greatly reduced cost, process time and labor, as compared to conventional methods heretofore employed. By use of a combination of cationexchange and anion-exchange resins, it is possible to remove all of the soluble ions from the emulsion and replace them with hydrogen and hydroxyl ions which produce the equivalent amount of water.

The following reaction is typical of the salt formation and the desalting step accomplished by ion exchange process.

Salt Formation in Precipitation of Silver Halide Y- is an anion forming a water soluble silver salt. Z+ is a cation forming a water soluble halide salt.

By use of the ion-exchange procedure, the desalting efficiency of gelatin and similar emulsions is greatly increased. Another advantage is that varieties of gelatin of varying quality can be used in lieu of the high quality gelatin previously required for silver halide gelatin emulsions. It has been found possible to treat gelatin emulsions containing hardeners, such as chromates and azides, which heretofore have not been subject to processing by prior art washing procedures.

It is an object of this invention to provide a method for rapidly purifying silver halide emulsions.

It is afurther object to provide a method for desalting such emulsions by treatment with cation-exchange and anion-exchange resins.

It is a further object to provide a method for desalting silver halide gelatin emulsions produced from lowquality gelatins.

Another object is to provide a method for desalting such emulsions which contain hardeners.

Another specific object is to provide an improved process of making reproductions on light-hardenable silver emulsions having greater printing speed in which the sensitizing solutions have been processed through ion exchanged resin columns to desalt the solution and to remove contaminators from the colloidal materials.

These and other objects are apparent from and are achieved in accordance with the following disclosure in conjunction with the attached drawing.

DESCRIPTION OF PREFERRED EMBODIMENTS The ion-exchange resins used in this invention are of two types. The first type is a cation-exchange resin with exchangeable hydrogen ions attached thereto, and the second type is an anion-exchange resin with exchangeable hydroxyl ions attached thereto. As used in this invention, the cationic resin, that is the resin with hydro gen ions attached thereto, replaces the cations in the emulsion such as alkali metal ions, alkaline earth metal ions and other metallic ions, with hydrogen ions. Subsequently, in the operation, the anion-exchange resin, that is the resin containing hydroxyl ions thereon, provides hydroxyl ions which replace the anions in the emulsion such as halide, nitrate, sulfate, phosphate and related anions. During the course of the operation the ion-exchange resins become depleted of hydrogen and hydroxyl ions, respectively, and must be regenerated by treatment with acid and with caustic soda or similar alkali, respectively.

Synthetic cation-exchange resins suitable for use in this invention are prepared by known procedures by sulfonating a polymer, for example. polystyrene or a copolymer of styrene and divinylbenzene, to produce a resin containing sulfonic acid (SO H) substituents.

These substituents provide hydrogen ions and take up other cations. By analogous known procedures, anionexchange resins can be produced which provide hydroxyl ions. These are usually resins which contain quaternary ammonium hydroxide substituents of the general formula NRR'R"OH wherein R, R and R" are organic radicals such as alkyl (e.g. methyl), attached to the resin unit which provide hydroxyl ions to replace other anions. The most common substituent in such resins is the trimethyl-ammonium hydroxide radical, N(CH OH. One particularly desirable anionexchange resin of the latter type which provides hydroxyl ions is known as 1RA93 produced by Rohn & l-Iaas Company. Ion-exchange resins of both types described above are well known and commercially available.

In the operation of this invention, the aqueous emulsion containing the precipitated silver halide and extraneous ions is passed through a strongly acidic cationexchange resin which provides hydrogen ions to replace the cations and then through a strongly or moderately basic anion-exchange resin providing hydroxyl ions to replace the anions. The resins may be arranged in reverse order with the hydroxyl-ion-producing resin first, and the resin providing hydrogen ions second, or the resins may be mixed together in the column. By this procedure, the metallic cations and the soluble anions are removed and replaced by hydrogen and hydroxyl ions, respectively, resulting in deionization of the emulsion.

The ion-exchange operation can be carried out in conventional equipment, such as columns, tanks or kettles. One column can contain the basic resin and one can contain the acidic resin, or the two types of resin can be combined in the same columns. Moreover, a mixed bed of acidic and basic resins may be used. In kettles, batch treatment is suitable, followed by decantation or filtration to remove the colloidal dispersion from the ion-exchange resins. The flow of the dispersion through the resin is operable in any direction, but the preferred operation is to circulate the emulsion upward through the resin bed and then regenerate the exhausted resin by circulation of the regeneration fluid downward through the bed.

The preferred temperature range for the operation is from 16 to 60 C. Ordinarily, ambient temperature from 20 to 25 C is preferred. In actual practice, the operation may be conducted at temperatures from the freezing point of the emulsion up to the decomposition temperature of the emulsion or resin, but generally such high temperatures are avoided.

Ordinarily, the emulsion is circulated by gravity feed but pressures (hydraulic or gas) of l to 50 psig. are operative. The flow rates are usually governed by the particle size of the resins and the viscosity of the emulsion. The preferred rate is between 0.5 and 4 gallons per minute per cubic foot of resin, but rates up to 25 gal./min./cu.ft. are operative. By this procedure, extraneous ions in gelatin emulsions can be removed in ten minutes or less. Then conventional components are added to the emulsion. An emulsion thus prepared is coated on a support and dried. The support, usually of l 8 mil. gauge, may be coated and/or treated cloth, paper, or plastic film such as cellulose ester, polystyrene or polyester.

Although a variety of layers may be coated on the support, the outermost layer to receive the emulsion is of essentially hydrophilic character, often consisting of a water-soluble colloid of the same type as used in the emulsion.

In use, the coated emulsion is exposed imagewise to light and then wet-processed to wash off the emulsion from unexposed areas to produce a silver image on the support, and then dried.

The basic formulation of such emulsions comprises the following:

Parts by weight colloid l-l5 soluble silver salt 0. l-20 soluble halide salt 0.05-25 water The colloid may be gelatin, glue or a synthetic watersoluble polymer such as polyvinylpyrrolidone or polyvinyl alcohol. Water-dispersible colloid mixtures may also be used. Suitable colloids are disclosed in Reichel et al. U.S. Pat. No. 2,663,640 and Yackel U.S. Pat. No. 2,592,368.

The halide is usually an alkali metal chloride, bromide, or iodide, or mixtures thereof. However, the anion may be other than an alkali metal ion; e. g. it may be nickel, cadmium, etc.

In addition to the precipitated emulsion there are many additives which may be incorporated, such as stabilizers, antifoggants, plasticizers, developer precursors, pH buffers, sensitizers, wetting agents, hardeners, etc. which are well known in the art. Usually additives would be incorporated after the ion-exchange step.

The order and mode of addition of components may vary widely; e. g., the hardener may be incorporated in the emulsion or in a subsequent processing bath, or more colloid may be added after ion exchanging. The variations used in emulsion making and the selection of formulation depend on the application.

However, one needs only the basic formulation described above to demonstrate the invention within operating ranges used in the art, as set forth below.

OPERATING RANGES Temperature of coating solution 25 70 C Drying temperature 25 C, but not to exceed the softening point of the substrate Viscosity not critical, l-500 cps.

Amount of water sufficient dilution to obtain a coating weight of 0.5 8 mg/sq.in.

Moisture in dried coating 4 20% Other cation-exchange resins which can be used in this invention include sulfonated styrenedivinylbenzene copolymers, known under the trademark Dowex 50X1, polymethacrylic acid known as Amberlite IRCSO, and polyacrylic acid known as Amberlite XE89. These resins provide hydrogen ions and remove cations from the emulsions.

Other anion-exchange resins which can be used include trimethylhydroxyaminomethylated styrenedivinylbenzene copolymer produced by reaction of chloromethylated styrene-divinylbenzene copolymer with trimethylamine and exchange of the chloride ion with hydroxide, known as Amberlite IRA402, Amberlite IRA93 and Dowex lXl, and phenol-formaldehyde trimethylammonium hydroxide resins known as Amberlite IR4B.

Ion-exchange resins for monobed systems include Amberlite MBl and Amberlite MB3 which are mixtures of anion-and cation-exchange resins of the type described above. Other suitable mixtures include Amberlite IR120 and Amberlite IRA402, and Amberlite [R200 and Dowex 1X1.

In the batch operation, the emulsion to be desalted is mixed with the ion-exchange material and the mixture is agitated for l;- l5 minutes. In one convenient procedure the ion-exchange material is in a cloth bag which is immersed in the emulsion and then removed. In any procedure, the resin is finally separated from the emulsion by filtration, decantation or centrifugation. Then the process is repeated with another ionexchange material to remove the cations as well as'the anions. The ion-exchange materials are washed free of any residual emulsion and regenerated from time to time.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates the layout and arrangement for a commercial two-bed ion-exchange desalting system for a photographic emulsion. The emulsion is produced in the reaction kettle from which it is withdrawn via line 12, pump 14 and line 16 to the desalting equipment. The emulsion is passed through one of the cartridge filters 18 and 20 via appropriate lines 22, 24 and 26 and valves 28, 30 and 32 and thence into reaction vessel 36. This vessel is charged with a supply of cation-exchange resin from the tank 38. The emulsion and cation-exchange resin in vessel 36 are agitated for an appropriate period of time, separated and the emulsion passed by lines 40, 42 and 44 and valve 46 into the second reaction vessel 48 which is charged with anionexchange resin from tank 50. The resin and emulsion are agitated together, then separated, and theemulsion withdrawn via line 52. It passes through a conductivity cell 54 equipped with a light 56 which measures the electrical conductivity and therefore the salt (ion) content of the emulsion, and indicates when the ion content is too high by causing the light 56 to be energized. If the conductivity of the resulting emulsion meets the test, the desalted emulsion is passed through valve 58 and line 60 to the collector vessel 62. The ion-exchange resins from the vessels 36 and 48 are returned to the re- EXAMPLE I A simple photographic emulsion of the type described by Baker (Photographic Emulsion Techniques", 2nd ed., American Photographic Publishing Co., Boston, 1948) may be used to demonstrate the invention.

Parts gelatin, 250 bloom, acid, hide 2 silver nitrate 2.5

sodium chloride 1 water 100 The above precipitated mixture is then ionexchanged according to the method of this invention in about 110 minutes. It is circulated through a column packed with a strongly acidic ion-exchange resin (Amberlite IR12O or IRZOO of Rohm and Haas Company) f at the rate of 2 gal./min./cu.ft. of resin, then through a column of basic resin (Amberlite IRA93 of Rohm and Haas Company) at the same rate. This emulsion is then completed as recommended by Baker with about 50 cc. of 40 percent formaldehyde solution per kg. of gelatin and a small amount of saponin wetting agent.

EXAMPLE 2 A light tanning emulsion such as described by Kosar (Light-Sensitive Systems", John Wiley & Sons, Inc., New York, 1965, pp. 114 118) is a useful example for demonstrating the invention.

water 40 Part (I) of the above mixture is ion-exchanged for 10 minutes according to the method of Example 1 and the components of Part (II) are added.

EXAMPLE 3 An emulsion is made as follows:

To a mixture of 5 liquid ounces of Le Pages photographic glue, and 77 liquid ounces of distilled water, are slowly added dropwise while under constant stirring 9 fluid ounces of silver nitrate solution (3.07 ounces by weight of dry salt in distilled water) and 9 fluid ounces of potassium chloride solution (1.43 ounces by weight of potassium chloride dissolved in distilled water). To this milk-like emulsion is added 1 lb. of mixed cationand anion-exchange resins (Amberlite MBl) and the mixture agitated slowly for 20 minutes. It is then decanted and the liquid filtered through cheesecloth. Then 2 oz. of sodium 4,4'diazidostilbene-2,2'- disulfonate in 10 fluid oz. of water are added with stirring. The emulsion is ready for application to a base.

EXAMPLE 4 A method of making an emulsion suitable for use in sensitizing a tracing cloth is as follows:

Solutions are made of (A) 25 grams of gelatin in 1 liter water at 40 C, (B) grams silver nitrate in 500 cc. water at 20 C and (C) 35 grams of sodium chloride in 500 cc. of water. Solutions 13 and C are simultaneously run into Solution A at a uniform rate while stirring the latter over a period of about 10 minutes; Solution B preferably not being allowed to run in faster than C. Thereafter, 150 grams of gelatin in 1,500 cc. of water at 40 C are added. The emulsion is then passed through a vertical glass tube (1 inch LD. and 2 feet long) packed with particles of cation-exchange resin (Amberlite IR120) and then through a similar vertical tube packed with anion-exchange resin (Amberlite IRA402). The flow rate is adjusted so that the emulsion passes through both tubes in 10 minutes. The flow can be either downward or upward through the resin and the order of the two resins can be reversed.

After the deionization, the emulsion is adjusted to pH 5 and a solution of 25 grams of 3,4-dihydroxybiphenyl in 350 cc. of methanol is added slowly with stirring. Then 2 cc. of 10 percent formalin solution is added to the emulsion and it is coated on a tracing cloth over an area of about 500 sq. ft. and dried. The dried product is exposed to a suitable negative and developed in a 4 per cent sodium carbonate solution. The developed element is then washed with water at 35 C. to remove the unexposed, undeveloped and untanned emulsion, is then rinsed and dried. A positive relief image in gelatin is thus obtained with no gelatin remaining in the unexposed area. Erasures and redrawing of lines were as satisfactory on the print as on the uncoated tracing cloth. Curl was greatly diminished and was approximately equivalent to that of the uncoated tracing cloth, after having been processed through the same baths.

EXAMPLE A photographic emulsion containing poly-N-vinyl caprolactam having an average molecular weight of about 40,000 as the binder for silver halide was prepared as follows:

Separate aqueous solutions containing the following constituents were made.

A. AgNO;(3N) 290 cc Nl-LOH (20%) 290 cc Water 70 cc B. NH Br (3N) 310 cc Kl (0.5M) 20 cc Water 320 cc C. solution of poly-N-vinyl caprolactam in water 1000 grams These three solutions were mixed simultaneously by running streams of solutions A and B from separate containers into a common reaction vessel containing solution C with rapid agitation. The resulting lightsensitive silver halide suspension in the poly-N-vinyl caprolactam was desalted by passage through a 6 inch bed of mixed cation-exchange and anion-exchange resins (Amberlite [R200 and Dowex 1X1), the contact time with the resin bed being minutes. Coatings of this emulsion were made on raw stock paper coated with baryta gelatin. After drying, the photographic element was exposed by focusing on it the image of a transparency negative and developed by immersion at C in a standard p-methylamino-phenoL hydroquinone developer having the following compositron:

Parts p-Methylaminophenol sulfate 3.1 Sodium sulfite (anhydrous) 45.0 Hydroquinione 12.0 Sodium carbonate (anhydrous) 67.5 Potassium bromide 1.9 Water to make L000 The developed picture was fixed by bathing in a percent solution of sodium thiosulfate, freed from soluble salts by washing, and then dried. The resulting black and white picture had density gradations complementary to the object scene to which the element was exposed. The developed picture formed from the poly- N-vinyl caprolactam showed speed contrast, and fog values comparable to that of commercial gelatin-silver halide emulsions used on enlarging papers. Prints on paper of this emulsion have a pleasing warmth of tone.

Similarly, a photographic emulsion can be prepared in which poly-N-vinyl caprolactam is used as a binder for light-sensitive silver halide and a color former. After exposing and processing in the usual manner with a color developer containing N-p-diethylaminoaniline, a photographic element bearing a colored photographic image can be obtained.

In each example the effectiveness of ion-exchange desalting may be verified by pH and nitrate tests or by simply coating the ion exchanged emulsion on a glass plate and comparing with a like emulsion which was not desalted. The non-desalted emulsion coating is marred by defects due to crystallized salts while the ion-exchanged emulsion is free of these defects.

In ion-exchanged emulsions with glue or gelatin colloid, coatings usually have increased printing speed compared to unwashed emulsions or emulsions washed by conventional means. Reference is made to the sensitizing emulsion of Example 2. For purposes of illustrating the unique advantages of the instant invention, the sensitizing solution was separated into two portions; number one was processed through the ion exchange system and the number two portion coated on a polyester ester after processing through conventional desalting techniques by washing the precipitated colloid with cold water for 30 minutes.

Films prepared using the number two solution prepared by prior art techniques and exposed to a film negative in a Macbeth and lamp rated at 60 amperes at a distance of 42 inches required a 40 second exposure. The film prepared with the number one solution required a 20 second exposure.

The film exposed for 3 minutes produced a film in which the image portion was suitably hardened and the print density after development in the sulfate bath was 1.5 densitometer units. The non-hardened portion of the coating was washed off leaving a transparent film reproduction with opaque portions.

The 20 second exposed film gave similar image density and tended to wash off more easily.

In general, exposure speed increases are realized from producing this invention in the range of from 30 to percent.

With emulsions of the light hardening type, such as of Example 2, this is particularly easy to demonstrate since exposure times are generally quite long and improvement in speed by ion-exchanging can be easily detected.

This effect of increased speed is attributed to removal by ion exchange of components characterized by light absorption in the 250 400 millimicron wavelength region. The degree of speed increase depends on the purity of the colloid used. UV-visible spectra of the raw gelatin gives an indication of the impurities present.

Analysis showed that in general the light transmittance of gelatin in the 250 400 millimicron region increased when treated according to the method of the invention. Moreover, the components removed, when retrieved from the ion exchange by displacement were found to interfere with printing speed when added back synthetically to emulsion, especially when the emulsions were of high spectral sensitivity in the 250 400 millimicron region.

While there has been illustrated and described a sin gle embodiment of the present invention, it will be apparent that various changes and modifications thereof will occur to those skilled in the art. it is intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. The method of making a reproduction on a silver sensitized, light hardenable material comprising the steps of forming a sensitized member by combining a silver salt and a halide salt in a colloid-aqueous solution to produce a silver halide and salt, contacting said colloidal solution with a first ion-exchange resin which provides one of the ions selected from the class consisting of hydrogen ions and hydroxyl ions and a second ion-exchange resin which provides the ion absent from said first exchange resin, the time of contact of said emulsion with lack of said ion-exchange materials being sufficient to exchange substantially all of the ions in said emulsion for hydrogen and hydroxyl ions,

separating the emulsion from the ion-exchange materials,

adding a hardening agent to said colloidal solution,

apply the solution to a suitable base support and removing the excess water by drying,

exposing the coated base support to actinic radiation through a patterned transparency causing the colloidal material to light-harden and become water insoluble in the areas struck by said actinic radiation,

developing in a developing solution of the reducing type to produce a visible silver image, and washing the developed member in water to remove light-sensitive, non-hardened emulsion.

2. The method as claimed in claim 1 wherein the colloid is gelatin and the hardening agent is a chromate ion containing salt.

3. The method as claimed in claim 1 wherein the colloid is glue and hardening agent is 4.4'-diazidostilbene- 2, 2'-disulfonate.

4. The method as claimed in claim 1 wherein the colloid is gelatin and the hardening agent is 3,4- dihydroxybiphenyl.

5. The method as claimed in claim 1 wherein the range of components in parts by weight comprising the sensitized coating solution is:

Colloid 5 10 Silver Nitrate 5 10 Potassium Chloride 2.5 5 Water 6. The method as claimed in claim 1 wherein the amount of hardening agent added is in the range of 0.5

to 1.0 parts per 25 parts of colloid. 

2. The method as claimed in claim 1 wherein the colloid is gelatin and the hardening agent is a chromate ion containing salt.
 3. The method as claimed in claim 1 wherein the colloid is glue and hardening agent is 4.4''-diazidostilbene-2, 2''-disulfonate.
 4. The method as claimed in claim 1 wherein the colloid is gelatin and the hardening agent is 3,4-dihydroxybiphenyl.
 5. The method as claimed in claim 1 wherein the range of components in parts by weight comprising the sensitized coating solution is: Colloid5 - 10Silver Nitrate5 - 10Potassium Chloride2.5 - 5Water100 - 150
 6. The method as claimed in claim 1 wherein the amount of hardening agent added is in the range of 0.5 to 1.0 parts per 25 parts of colloid. 